1. Field of the Invention
This invention relates to an efficient process for desalination of salt water, particularly sea water, and production of fresh water. The invention is particularly concerned with a process and system of the above type for recovery of fresh water from salt water or sea water by direct contact heat transfer between hot water and an immiscible liquid, particularly a liquid hydrocarbon of low vapor pressure, to cause boiling thereof, and effecting direct contact heat transfer between the vapor of such immiscible liquid and the salt water to condense such vapor and heat the salt water, employing an arrangement of staged evaporator-condenser units for carrying out such boiling and condensation operations, and utilizing the heated salt water to generate fresh water.
2. Prior Art
Desalination is a growing industry in many parts of the world. Not only the countries with vast areas of arid lands, but the developed and the developing countries also are increasingly producing fresh water by desalination to meet the demands of growing population and rising standards of living.
Multi-stage flash and multi-stage evaporation are the most important processes currently in use for desalination of sea water. These processes suffer from two major disadvantages. In the first place, both require large metallic heat transfer surfaces. The cost of the heat transfer surface for these processes is about 35 to 40% of the total capital investment. Also, the corrosion and scaling of these surfaces are difficult to avoid, thus further increasing the cost by the need for replacement of corroded metallic surfaces. Secondly, the cost of energy requirements for these processes is relatively large, of the order of about $2.00 per 1,000 gallons of fresh water produced. Thus, any desalination process which eliminates, partly or wholly, the need for metallic heat transfer surface and/or requires less energy or a lower quality energy has attractive advantages.
A number of other methods and systems are also in use or being developed for desalination. These latter processes are based on the principles of vapor compression, reverse osmosis, freeze crystallization and ion exchange. All of these latter processes are relatively less attractive.
The improved processes described in the Smith U.S. Pat. Nos. 3,640,850 and 3,856,631 are based on heat transfer with direct contact between immiscible fluids. Hence these systems do not require metallic heat transfer surfaces, and such processes can operate with smaller amounts of energy per unit of water produced and with relatively low quality heat.
In the Smith patents hot sea water is flashed in a flash chamber and the water vapor generated is condensed in direct contact with fresh water. The hot fresh water is now brought in contact with a hydrocarbon liquid, which is immiscible with water. The hydrocarbon evaporates and the vapor is condensed in contact with sea water which is heated due to transport of latent heat released from the hydrocarbon vapor, and the heated sea water is heated further by an external heat exchanger. The hot sea water then enters the flash chamber. Although the use of metallic heat transfer surfaces thus is virtually eliminated, this design involves the flow of hydrocarbon and water in opposing directions and in contact with each other, the hydrocarbon following a substantially horizontal flow path between a plurality of evaporator and condenser units.
Other related but less pertinent prior art is set forth below.
The El-Roy patent, U.S. Pat. No. 3,337,421 shows a multi-stage system in which vaporized hydrocarbon is condensed by direct contact with a saline stream. However, the hydrocarbon is vaporized by indirect heat exchange.
Guptill et al in U.S. Pat. No. 3,392,089 discloses liquid-liquid heat exchange between a hot fresh water stream or condensate from a multiple effect evaporator, and a hydrocarbon stream. The heated hydrocarbon is not vaporized as it is chosen to have a high vapor pressure, and is used to transfer heat to a saline stream, by liquid-liquid heat exchange, and the preheated saline stream is fed to the multiple effect evaporator.
U.S. Pat. No. 3,446,711 discloses the condensation of steam by direct contact with a colder liquid hydrocarbon. The heated hydrocarbon is then passed in liquid-liquid exchange with a cold saline feed stream.
Woodward in U.S. Pat. No. 3,219,554 discloses liquid-liquid heat exchange between a hot fresh water stream and a hydrocarbon and between the heated hydrocarbon and an incoming saline stream. The hydrocarbon has a sufficiently high vapor pressure to preclude any significant vaporization.
U.S. Pat. No. 3,232,847 to Hoff employs a high boiling hydrocarbon which is passed counter current in liquid-liquid exchange to brine in a heating section and is used as a direct contact condensing medium for steam in a second section.
Osdor, U.S. Pat. No. 3,741,878 discloses a similar system in that a low vapor pressure hydrocarbon is used as a heat exchange medium.
It is an object of the present invention to provide an improved process and system for desalination of salt water or sea water, employing direct contact heat exchange. A further object is the provision of a process and system of the foregoing type, employing liquid-liquid direct contact heat exchange between an immiscible liquid and hot water to vaporize such liquid, and passage of such vapor into direct contact with the salt water to condense the vapor and transfer heat to the salt water. A particular object is to provide a process and system of the aforementioned type having high heat transfer coefficients due to intimate mixing between the two liquid phases, to promote boiling at low temperature differential between water and immiscible liquid or hydrocarbon, and hence improved thermal efficiency, and greater economy due to utilization of low quality energy and optimum selection of pressures in the staged system.